The present invention provides for methods for preparing an adsorbent containing sheet, their preparation into adsorbent monoliths and their subsequent use in air separation processes. More particularly, the present invention relates to the use of alkali or alkaline metal salts or mixtures thereof in preparing zeolite containing adsorbent sheets which can be used to construct monolith structures.
Cyclic adsorption processes are frequently used to separate the components of a gas mixture. Typically, cyclic adsorption processes are conducted in one or more adsorbent vessels that are packed with a particulate adsorbent material which adsorbs at least one gaseous component of the gas mixture more strongly than it adsorbs at least one other component of the mixture. The adsorption process comprises repeatedly performing a series of steps, the specific steps of the sequence depending upon the particular cyclic adsorption process being carried out.
In any cyclic adsorption process, the adsorbent bed has a finite capacity to adsorb a given gaseous component and, therefore, the adsorbent requires periodic regeneration to restore its adsorption capacity. The procedure followed for regenerating the adsorbent varies according to the process. In VSA processes, the adsorbent is at least partially regenerated by creating vacuum in the adsorption vessel, thereby causing adsorbed component to be desorbed from the adsorbent, whereas in PSA processes, the adsorbent is regenerated at atmospheric pressure. In both VSA and PSA processes, the adsorption step is carried out at a pressure higher than the desorption or regeneration pressure.
A typical VSA process generally comprises of a series of four basic steps that includes (i) pressurization of the bed to the required pressure, (ii) production of the product gas at required purity, (iii) evacuation of the bed to a certain minimum pressure, and (iv) purging the bed with product gas under vacuum conditions. In addition a pressure equalization or bed balance step may also be present. This step basically minimizes vent losses and helps in improving process efficiency. The PSA process is similar but differs in that the bed is depressurized to atmospheric pressure and then purged with product gas at atmospheric pressure.
As mentioned above, the regeneration process includes a purge step during which a gas stream that is depleted in the component to be desorbed is passed countercurrently through the bed of adsorbent, thereby reducing the partial pressure of adsorbed component in the adsorption vessel which causes additional adsorbed component to be desorbed from the adsorbent. The nonadsorbed gas product may be used to purge the adsorbent beds since this gas is usually quite depleted in the adsorbed component of the feed gas mixture. It often requires a considerable quantity of purge gas to adequately regenerate the adsorbent. For example, it is not unusual to use half of the nonadsorbed product gas produced during the previous production step to restore the adsorbent to the desired extent. The purge gas requirement in both VSA and PSA processes are optimization parameters and depend on the specific design of the plant and within the purview of one having ordinary skill in the art of gas separation.
Many process improvements have been made to this simple cycle design in order to reduce power consumption, improve product recovery and purity, and increase product flowrate. These have included multi-bed processes, single-column rapid pressure swing adsorption and, more recently, piston-driven rapid pressure swing adsorption and radial flow rapid pressure swing adsorption. The trend toward shorter cycle times is driven by the desire to design more compact processes with lower capital costs and lower power requirements. The objective has been to develop an adsorbent configuration that demonstrates a low pressure drop, a fast pressurization time and an ability to produce the required purity of oxygen.
For the details as to the manufacture of adsorbent sheets and the construction of monoliths used for dehumidification purposes, reference is made to U.S. Pat. Nos. 5,660,048, 5,660,221, 5,685,897, 5,580,369 and 4,012,206. The adsorption wheels are fabricated from an adsorbent paper which contains a adsorbent material. The adsorbent paper is prepared from a natural or synthetic fiber material. This fiber material can be combined with the adsorbent and wet-laid into a continuous sheet or hand sheet. This wet-laying is achieved by forming a slurry of the fiber, the adsorbent and binder components in water. This slurry is then transferred to a hand sheet mold or to the head box of a continuous wire paper machine for introduction onto the Fourdrinier or Twin-Wire paper machine. The adsorbent can contain zeolites, silica gels and/or alumina.
In a typical papermaking process, plenty of water from rivers, lakes or municipality supplies is used. When zeolite needs to be incorporated into the paper, these large quantities of water will have two negative effects on the performance of zeolite in a separation process. The first arises from the ion exchange of cations between the process water and zeolite and second one is the hydrolysis of zeolite cations leading to a protonic exchange. If the papermaking process is carried out at higher temperatures, the hydrolysis of zeolitic cations will be even more significant. Depending on the type of separation or catalytic application, the modification in chemical composition of zeolite can have dramatic effect on the performance.
Li-containing molecular sieves are widely used in air separation process to selectively adsorb N2 over O2, thereby producing O2 in a continuous process in a multiple bed operation. If the process water contains cations such as Na, Ca and Mg, which are found in typical water supplies the exchange of these cations into Li-sieve will lead a decrease in the Li content of zeolite and consequently a decrease on the sorption capacity of N2 and a decrease in N2/O2 selectivity. However, if the sorbate molecule has stronger interaction with cations, such as water as in a dehumidification application, the partial ion-exchange of zeolite cations from the process water may not have significant influence on the performance of the adsorbent. Hence, in an adsorption process involving weaker interactions, retaining the preferred original composition of the zeolite during the papermaking process is very crucial.
The present invention provides means to keep the original composition of the adsorbent by preventing the leaching of cations from the zeolite both during the preparation of stock preparation, and during and after the manufacture of the paper. It also describes a method for decreasing the hydrolysis of zeolite cations, when a slurry containing zeolite is coated or impregnated on to various substrates.
The present invention provides for an improved method for preparing an adsorbent containing sheet by the addition of alkali and alkaline metal salts or mixtures thereof to the adsorbent containing slurry sheet. These salts can be added to either the slurry during its preparation or by post-treating the monolithic structures with an aqueous solution of the metal salt. It also discloses using deionized water in place of river or lake water, which were typically used in paper industry.
The present invention further provides for an improvement by adding the metal salt to a zeolite containing slurry which will be coated to a substrate such as a flat sheet, corrugated sheet, metal foil or mesh.
The monolith adsorbents that have been prepared in this manner demonstrate improved nitrogen capacity and nitrogen/oxygen selectivity in air separation processes such as vacuum swing adsorption (VSA) or pressure swing adsorption (PSA) processes.